1. Field of the Invention
The present invention relates to an optical waveguide device such as optical fiber and an optical device equipped therewith, which are used in the field of optical communications and the like. In particular, the optical waveguide device is provided with long-period gratings which is suitable for compensating for the temperature dependence of gain in rare-earth-doped optical fiber amplifiers, and the like.
2. Related Background Art
A typical optical fiber communication system comprises a signal light source, an optical fiber line having one end optically coupled to the signal light source, and an optical receiver optically coupled to the other end of the optical fiber line. Disposed within the optical fiber line is an optical amplifier for amplifying the signal light propagating therethrough. Such an optical fiber communication system mainly uses signal light in the wavelength band of 1.5 .mu.m, and employs, as the optical amplifier, an optical fiber amplifier including an optical fiber doped with a rare-earth element such as erbium (Er).
In the above-mentioned optical fiber amplifier, when signal light in the 1.5-.mu.Mm wavelength band is made incident on the rare-earth-doped optical fiber in which population inversion has been formed upon incidence of excitation light having a predetermined wavelength, then it causes induced emission. The optical fiber amplifier amplifies the incident signal light by utilizing the induced emission.
In such an optical fiber amplifier, since the state of population inversion formed upon incidence of excitation light changes depending on temperature, fluctuations in the state of population inversion would cause the gain to vary or the noise figure to increase. Namely, when optical amplification is repeated by a plurality of optical fiber amplifiers, amplification-wavelength characteristics may vary in response to the fluctuating population inversion state, thus yielding different gains for their respective wavelength components (wavelength dependence of gain). In particular, in an optical communication system of wavelength-division multiplexing (WDM) type, individual channels (corresponding to the individual wavelength components included in the WDM signal) may attain different gains, thereby some channels may yield a high bit error rate.
In order to overcome such problems, techniques employing fiber gratings are disclosed, for example, in Japanese Patent Application Laid-Open No. 07-283786 and the paper titled "Long-Period Fiber Gratings as Band-Rejection Filters" (JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 14, No. 1, January 1996). The fiber grating is an area, formed within the core along the axis of an optical fiber, in which refractive index periodically changes. Though it encompasses Bragg gratings having a relatively short period and gratings having a longer period (about 50 to 1500 .mu.m in practice), long-period gratings are used in the above-mentioned paper.
As clearly shown in U.S. Pat. No. 5,703,978, the long-period gratings are gratings which induce coupling (mode coupling) between a core mode and a cladding mode by which light propagates through an optical fiber, and are clearly distinguished from short-period gratings which reflect light having a predetermined wavelength. For attaining a strong power conversion from the core mode to the cladding mode, the grating period (pitch) of long-period gratings is set such that the optical path difference between the core mode and the cladding mode is 2.pi.. Since the long-period gratings thus act to couple the core mode to the cladding mode, the core mode would attenuate over a narrow band centered at a predetermined wavelength (hereinafter referred to as "loss wavelength"). The above-mentioned paper states that, when a long-period grating having an optical attenuation characteristic corresponding to the wavelength distribution of amplified spontaneous emission is provided in an optical fiber line, then the part corresponding to the amplified spontaneous emission can be canceled, so as to flatten the wavelength spectrum of the amplified signal light.
Such a long-period grating is usually obtained by locally irradiating an optical fiber having a photosensitive core with light at predetermined intervals along the axis of the optical fiber, thereby generating a periodic optically-induced refractive index change therein. A prevailing method comprises the steps of preparing a silica-based optical fiber whose core is doped with germanium oxide or phosphorus which is a photosensitive material, placing on the optical fiber an intensity modulation mask in which light-transmitting portions and light-shielding portions are alternately arranged like a grating, and irradiating the optical fiber with an ultraviolet beam having a wavelength ranging from about 193 to 248 nm by way of the intensity modulation mask. According to this method, the ultraviolet light passing through each light-transmitting portion of the intensity modulation mask irradiates the optical fiber, thereby the refractive index would increase locally at the locations where the core doped with germanium oxide is irradiated with the ultraviolet light. As a result, an area, i.e., a grating, where the refractive index has been changed with a period substantially identical to the period with which the light-transmitting portions are arranged in the intensity modulation mask is formed within the core.
The center wavelength of the wavelength spectrum of the light emitted from a core to a cladding by a long-period grating, i.e., loss wavelength, is given by the following expression: EQU .beta..sub.core.sup.(lm) -.beta..sub.cladding.sup.(n) =2.pi./.LAMBDA. (1)
where 1 and m define the order of the core mode (l=0, m=1 in the case of the fundamental mode LP.sub.01), .beta..sub.core.sup.(lm) is the propagation constant determined by (lm), .beta..sub.cladding.sup.(n) is the propagation constant of the n-order cladding mode, and .LAMBDA. is the period (pitch) of the long-period grating.
Since the propagation constants .beta..sub.core and .beta..sub.cladding are parameters depending on the loss wavelength, the loss wavelength of the long-period grating can be controlled when the grating period .LAMBDA. is adjusted, as can be seen from expression (1). Also, while .beta..sub.core depends on the effective refractive index of the core, .beta..sub.cladding depends on the effective refractive index of the cladding, thereby, assuming the period of the grating to be constant, the loss wavelength of the long-period grating mainly depends on the relative refractive index difference between the core and cladding in the grating forming area (refractive index fluctuating area) in which the long-period grating is formed. Here, from the average value n.sub.ave of the fluctuating refractive index of the core and the refractive index n.sub.cld of the cladding, the relative refractive index difference .DELTA.n between the core and cladding in the grating forming area is given by the following expression (2): EQU .DELTA.n=(n.sub.ave -n.sub.cld)/n.sub.cld (2)
Further, since the amount of change of refractive index in the core varies according to the amount of irradiation of ultraviolet light at the time when the grating is formed, forming the long-period grating by adjusting the amount of irradiation of ultraviolet light (adjusting the relative refractive index difference between the core and cladding) can eventually control the loss wavelength of the long-period grating as well.